US6984866B1

US6984866B1 - Flip chip optical semiconductor on a PCB

Info

Publication number: US6984866B1
Application number: US10/391,283
Authority: US
Inventors: Shahram Mostafazadeh; Joseph O. Smith; Matthew D. Penry
Original assignee: National Semiconductor Corp
Current assignee: National Semiconductor Corp
Priority date: 2003-03-17
Filing date: 2003-03-17
Publication date: 2006-01-10

Abstract

Semiconductor devices and methods for making semiconductor devices. The present invention allows a flip chip assembly to be used with an optical semiconductor device. The optical semiconductor flip chip is positioned over a hole in a PCB such that the imaging area of the optical semiconductor flip chip faces the hole. The hole allows the imaging area to be unobstructed by the PCB. Underfill material can be prevented from going into the hole by erecting a barrier on top of the PCB that surrounds the hole.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical semiconductors more specifically to mounting optical semiconductors on printed circuit boards.

2. Description of the Related Art

There are a wide variety of digital imaging devices that are currently commercially available. The imagers used in these devices typically take the form of an integrated circuit having a charge-coupled devices (CCD) and/or CMOS imagers. CCDs and CMOS imagers are specially made integrated circuits that respond to light. CCDs are used to capture image data in devices such as telescopes, bar code readers, digital still and video cameras and scanners.

Packaging assemblies are often costly aspect of optical semiconductor chips. The imagining area of an optical semiconductor chip must be generally remain unobstructed, which often requires expensive or intricate packaging designs.

One wafer level packaging approach that has been used in packaging integrated circuits having optical components is the chip-scale “ShellOP” type packaging technology developed by Shellcase Ltd. of Israel. A cross-sectional view of a typical ShellOP package is shown by way of example in FIG. 1. As seen therein, IC package 100 is an optically active device based on the ShellOP packaging design. The packaging process employs standard wafer processing techniques such as grinding, photolithography, etching, metal deposition, plating, and dicing. Unlike many packaging methods, the Shellcase process requires no lead frames, or wire bonding. The optical package comprises semiconductor bulk 105, which is held in placed in between a top glass plate 110 and a lower glass plate 115 by

epoxy

120 and 125, respectively. Inverted external leads 130 are electrically connected to die terminals 135 by trace contacts 140 at junctions 145. Junction 145 is sometimes referred to as a T-junction, and contact 140 as a T-junction contact. External leads 130 are coated with a protective solder-mask 150. Solder-mask 150 is a dielectric material that electrically isolates leads 130 from external contact, and protects the lead surface against corrosion. Contacts 155 are attached to the bottom end of leads 130, and are suitable for printed circuit board (PCB) mounting by known methods. Contacts 155 may be formed by known methods such as solder-balls or plating, and may be suitably shaped for PCB mounting.

Although the ShellOP and other existing packaging processes have proven to be useful, they are costly to implement. Other well-known packages are cheaper, but have not been used because the assemblies result in an obstructed imaging area. For example, although a flip chip optical semiconductor is cheaper to make, they would typically result in the imaging area being face-down and obstructed by the PCB.

SUMMARY OF THE INVENTION

The present invention provides semiconductor devices and methods for making semiconductor devices. In one embodiment, a semiconductor device includes a PCB, a optical semiconductor, underfill material and a barrier. The PCB, a substrate adapted to receive the optical semiconductor, has an opening that extends from the top of the PCB to the bottom of the PCB. The optical semiconductor is attached to the PCB such that the optical semiconductor's imaging area is facing the opening and substantially unobstructed by the PCB. The underfill material is dispensed in between the optical semiconductor and the PCB. The barrier is located in between the optical semiconductor and the PCB, such that the barrier surrounds the opening in the top of the PCB and is adapted to prevent the underfill material from entering the opening.

In another embodiment, a semiconductor device is constructed by first providing a PCB that has an opening that extends from the top of the PCB to the bottom of the PCB. Then a barrier is dispensed around the opening on the top of the PCB. Afterwards, a flip chip optical semiconductor can be coupled to the top of the PCB such that the flip chip optical semiconductor covers the opening. Then underfill material can be dispensed in between the flip chip optical semiconductor and the PCB, whereby the barrier prevents the underfill material from going into the opening. Finally, a transparent cover can be attached to the bottom of the PCB.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross sectional view of a prior art ShellOP package;

FIG. 2A is a plan view of a PCB with two openings;

FIG. 2B is a cross sectional view of the PCB illustrated in FIG. 2A;

FIG. 3A is a plan view of the PCB of FIG. 2 with barriers surrounding the two openings;

FIG. 3B is a cross sectional view of the PCB illustrated in FIG. 3A;

FIG. 4A is a plan view of two flip chips attached to the PCB of FIG. 3;

FIG. 4B is a cross sectional view of the assembly illustrated in FIG. 4A;

FIG. 5A is a plan view of the assembly of FIG. 4 with underfill material;

FIG. 5B is a cross sectional view of the assembly illustrated in FIG. 5A;

FIG. 6A is a plan view of the assembly of FIG. 5 with opaque protective layers and transparent covers; and

FIG. 6B is a cross sectional view of the assembly illustrated in FIG. 6A.

It is to be understood that, in the drawings, like reference numerals designate like structural elements. Also, it is understood that the depictions in the figures are not necessarily to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the present invention.

The present invention allows a flip chip assembly to be used with an optical semiconductor device. The optical semiconductor flip chip is positioned over a hole in a PCB such that the imaging area of the optical semiconductor flip chip faces the hole. The hole allows the imaging area to be unobstructed by the PCB. Underfill material can be prevented from going into the hole by erecting a barrier on top of the PCB that surrounds the hole.

FIG. 2A is a diagrammatic plan view of an exemplary printed circuit board (PCB) 205 that can be used in the present invention. FIG. 2B is a diagrammatic cross sectional view of the PCB 205. A PCB is a substrate on which chips and other electronic components are placed, interacting with each other through traces. The PCB 205 can be of any suitable material including fiberglass-resin laminate such as standard FR4, ceramic, or flex tape. The PCB 205 has two

openings

210 and 215 that will eventually be used to allow an optical semiconductor to have an unobstructed imaging area. The

openings

210 and 215 can be of any shape, but will typically be of the same shape as the optical semiconductors' imaging portions. It should be readily appreciated that the number of optical devices will depend upon the particular requirements of the final device. Some PCBs might only have a single optical semiconductor and other PCBs might have many more optical semiconductors.

The PCB 205 additionally has two rows of

landing pads

220, 225, 230 and 235 per

opening

210 and 215. Landing pads are the sites that are used to connect the optical semiconductors with the rest of the system via traces 240. Although two rows of

landing pads

220, 225, 230 and 235 are shown, the number and placement of landing pads will depend upon the specifications of the optical semiconductors.

FIG. 3A is a diagrammatic plan view and FIG. 3B is a diagrammatic cross sectional view of the PCB 205 after

barriers

305 and 310 have been deposited around the

openings

210 and 215. The

barriers

305 and 310 are preferably a B-stageable thermosetting resin.

B-stageable thermosetting resins have three stages: A-stage, B-stage and C-stage. The A-stage is the condition of low molecular weight of a condensation resin polymer during which the resin is readily soluble and fusible. The B-stage is the condition of a partially cured resin polymer when it is more viscous, with higher molecular weight than in the A-stage, being incompletely soluble but plastic and heat fusible. The C-stage is the condition of a resin polymer when it is completely polymerized (in the solid state), with maximum molecular weight, being insoluble and infusible.

Working with the

barriers

305 and 310 while they are still somewhat pliable are preferred because they allow a larger tolerance in the heights of the

barriers

305 and 310 than would be necessary if the

barriers

305 and 310 were rigid. A large tolerance is desirable to compensate for settling that may occur after optical semiconductors are electrically coupled to their

respective landing pads

220, 225, 230 and 235.

It should, however, be appreciated that precise controls of certain parameters will permit accurate prediction of settling and allow the final height requirement of the

barriers

305 and 310 to be accurately known. In processes where the final height requirements of the

barriers

305 and 310 are known, the barriers can be made of rigid material, or even formed as part of the PCB 205.

FIG. 4A is a diagrammatic plan view and FIG. 4B is a diagrammatic cross sectional view of two flip

chip imaging dice

405 and 410 that are electrically coupled to the PCB 205 via

conductive bumps

415 and 420. A flip chip assembly is generally defined as the direct electrical connection of face-down (“flipped”) electronic components onto substrates, such as PCBs, by means of conductive bumps on bond pads.

Common conductive bumps include solder bumps, plated bumps, stud bumps and adhesive bumps. The

conductive bumps

415 and 420 can serve several functions in the flip chip assembly. The

conductive bumps

415 and 420 provide the electrically conductive path from the flip

chip imaging dice

405 and 410 to the PCB 205. The

conductive bumps

415 and 420 can also provide thermally conductive paths to carry heat from the

dice

405 and 410 to the PCB 205. In addition, the

conductive bumps

415 and 420 also provide part of the mechanical mounting of the

dice

415 and 420 to the PCB 205.

FIG. 5A is a diagrammatic plan view and FIG. 5B is a diagrammatic cross sectional view of the PCB 205 after underfill

material

505 and 510 has been applied. The

underfill material

505 and 510 is typically a non-conductive adhesive joining the entire surface of the flip

chip imaging dice

405 and 410 to the PCB 205. The

underfill

505 and 510 protects the

conductive bumps

415 and 420 from moisture or other environmental hazards, and provides additional mechanical strength to the assembly. Additionally, the

underfill material

505 and 510 helps compensates for any thermal expansion differences between the flip

chip imaging dice

405 and 410 and the PCB 205. The

underfill material

505 and 510 mechanically “locks together” the flip

chip imaging dice

405 and 410 and the PCB 205, attempting to prevent the differences in thermal expansion from breaking or damaging the electrical connection of the

conductive bumps

415 and 420.

The

underfill material

505 and 510 may be dispensed in any suitable manner. By way of example, the

underfill material

505 and 510 may be needle-dispensed along the edges of each of the flip

chip imaging dice

405 and 410. The

underfill material

505 and 510 is drawn into the under-chip space by capillary action. However, because of the

barriers

305 and 310, the

underfill material

505 and 510 does not flow into the

openings

210 and 215. It should be appreciated that the

barriers

305 and 310 are capable of preventing the

underfill material

505 and 510 from passing into the

openings

210 and 215 even if the

barriers

305 and 310 do not extend all the way from the top of the PCB 205 to the bottom of the flip

chip imaging dice

405 and 410. This is because the viscosity of the

underfill material

505 and 510, is too high to flow into small spaces via capillary action, which is typically the only mechanism that causes the

underfill

505 and 510 material to flow. Therefore, the heights of the

barriers

305 and 310 do not need to be equal to the spacing between the flip

chip imaging dice

405 and 410 and the PCB 205.

Once the

underfill material

505 and 510 is in place, the entire assembly is cured to form a permanent bond. If a B-stageable thermosetting resin were used to form the

barriers

305 and 310, the curing process could additionally cure the

barriers

305 and 310 at the same time the

underfill material

505 and 510 was cured.

FIG. 6A is a diagrammatic plan view and FIG. 6B is a diagrammatic cross sectional view of the PCB 205 after opaque

protective layers

605 and 610 have been applied to the back surfaces of the flip

chip imaging dice

405 and 410 and

transparent covers

615 and 620 have been attached to the PCB 205. Imaging chips are typically very susceptible to photo-generated carriers when the surfaces of the imaging chips are exposed to light. In order to prevent photo-generated carriers from interfering with the normal operation of the chips, an opaque cover can be placed on the exposed surfaces of the chips. It will be appreciated by those skilled in the art that the opaque

protective layers

605 and 610 can be placed on the flip

chip imaging dice

405 and 410 at any point in the fabrication process, even during wafer processing. For example, U.S. Pat. Nos. 6,352,881 and 6,023,094, both of which are incorporated by reference in their entireties for all purposes, disclose methods of applying an opaque protective layer to the top surface and/or bottom surface of the flip chip die during wafer processing.

However, the

transparent covers

615 and 620 will typically be applied after the

underfill material

505 and 510 has been cured, unless a system is in place to prevent condensation on the

transparent covers

615 and 620 during heating. The transparent covers 615 and 620 can be glass or transparent plastic, and can be attached to the PCB 205 via a standard epoxy. Once attached, they protect the imaging portion of the flip

chip imaging dice

405 and 410.

Although illustrative embodiments and applications of this invention are shown and described herein, many variations and modifications are possible which remain within the concept, scope, and spirit of the invention, and these variations would become clear to those of ordinary skill in the art. For example, the barriers might be constructed on the chips and not the PCB. Additionally, instead of using a transparent cover, the opening might be filled with a transparent material. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.

Claims

1. A semiconductor device comprising:

an optical semiconductor leaving an imaging area;

a PCB having a top, a bottom, and an opening that extends from the top of the PCB to the bottom of the PCB, the optical semiconductor being mounted directly on the top surface of the PCB using one or more electrical contacts formed between the optical semiconductor and the top surface of the PCB, the optical semiconductor being mounted such that the imaging area is facing the opening and substantially unobstructed by the PCB;

an underfill material that is dispensed in between the optical semiconductor and the PCB; and

a B-stageable thermosetting resin barrier in between the optical semiconductor and the PCB, such that the barrier surrounds the opening in the top of the PCB and is adapted to prevent the underfill material from entering the opening.

2. The semiconductor device of claim 1, further comprising a transparent cover that is attached to the bottom of the PCB such that the transparent cover entirely covers the bottom of the opening.

3. The semiconductor device of claim 1, wherein the thermosetting resin is an epoxy.

4. The semiconductor device of claim 3 wherein the epoxy is deposited on the PCB in its B-stage.

5. The semiconductor device of claim 1 wherein the barrier is a rigid material.

6. The semiconductor device of claim 5 wherein the barrier is formed as part of the PCB.

7. The semiconductor device of 1 wherein the optical semiconductor is a flip chip.

8. The semiconductor device of claim 1 wherein the PCB is a flex tape substrate.

9. The semiconductor device of claim 1 wherein there is a gap between the barrier and the optical semiconductor.

10. A method for constructing semiconductor devices comprising:

providing a PCB having a top, a bottom, and an opening that extends from the top of the PCB to the bottom of the PCB;

mounting a flip chip optical semiconductor directly onto the top surface of the PCB using one or more contacts formed between the top surface of the PCB and the flip chip optical semiconductor; and

forming a barrier around the opening on the top of the PCB, the barrier comprising a B-stageable thermosetting resin.

11. The method of claim 10, further comprising:

positioning the flip chip optical semiconductor on the top of the PCB such that optical circuitry on the flip chip optical semiconductor is exposed through the opening of the PCB.

12. The method of claim 11, further comprising,

dispensing underfill material in between the flip chip optical semiconductor and the PCB, whereby the barrier prevents the underfill material from going into the opening.

13. The method of claim 12, further comprising:

attaching a transparent cover over the opening on the bottom of the PCB.

14. A semiconductor device comprising:

a PCB having, a top, a bottom, and an opening that extends from the top of the PCB to the bottom of the PCB;

a flip chip having optical circuitry mounted directly on the top surface of the PCB such that the optical circuitry on the flip chip is exposed through the opening in the PCB; and

a barrier adapted to prevent underfill material from entering the opening when the chip is coupled the PCB, the barrier comprising a B-stageable thermosetting resin.